Effect of daily systemic injections of donepezil (0, 0.5, 1, 1.5 mg/kg, ip) on tolerance to the analgesic effect of morphine. Each bar represents the mean of percentage of maximal possible effect (%MPE) ± sem for eight rats. Student’s t-test was used to analysis the differences of %MPE between saline and treated animals every day also it was utilized for comparison of the day of tolerance establishment in control or treatment groups. *P < 0.05; **P < 0.01; ***P < 0.001 when compared with the saline group. a = P < 0.001 when compared with the control group. Arrow represents the day of morphine tolerance. M = Morphine; DPZ = Donepezil.